Method of processing a photosensitive mosaic electrode



Jan. 1, 1957 R. E. HOFFMAN ETAL METHOD OF PROCESSING A PHOTOSENSITIVE MOSAIC ELECTRODE Filed Aug. 19, 1953 INVENTORS IP/CHH/PDE. HOFFMAN A,

W/LuA/vr 6. Roar Jay/425 JTTORNE Y METHOD OF United States Patent PROCESSING A PHOTOSENSITIVE MOSAIC ELECTRODE Richard E. Hoffman, Florin, and William G. Rudy, Lancaster, Pa., assignors to Radio Corporation of America, a corporation of Delaware Application August 19, 1953, Serial No. 375,247

5 Claims. (Cl. 117-210 This invention is directed to a cathode ray camera tube for television. More specifically, the invention is directed to a novel and improved target electrode and the method of making the photosurface of the target for a television camera pickup tube of the iconoscope type.

The iconoscope type of television camera pickup tube is one in which an electron gun forms a high velocity electron beam, which is scanned over a photosensitized target electrode. A charge pattern is established on the photosensitized surface of the target electrode by the light variations of an optical image focused thereon. The electron beam, in scanning over the charge pattern established on the target electrode, sets up a series of signal pulses in a conductive signal electrode, capacitively coupled to the photo-sensitized surface of the target. This series of signal pulses constitutes the signal output of the tube. Such a camera tube is similar to that described in U. S. Patent 2,189,985 to W. H. Hickok.

The target of such a tube may be made with a thin sheet of mica, upon one surface of which is formed a mosaic of photosensitized material. On the opposite surface of the mica sheet is applied a conductive coating of a suspension of carbon in a binder, or a commercial coating such as Aquadag.

Tubes of this type often have low sensitivity originally, or after a time of use lose both sensitivity and picture resolution. It is believed that these defects are due to the presence of cesium metal vapor used within the tube for sensitizing the photocathode. The photosensitive mosaic formed on the target as a photocathode, is a coating of minute globules of silver metal which are insulated by the mica sheet from each other. The silver is oxidized and then photosensitized by condensing on each globule a thin film of cesium metal vapor. However, during this process, the cesium metal also is deposited between the silver oxide globules and establishes conductive paths therebetween, which effectively short the globules to each other. The amount of conductivity between globules inversely efiects the sensitivity of the photocathode. Furthermore, it has been found that during tube operation, cesium metal within the evacuated tube tends to drift from adjacent surfaces and is deposited on the target electrode. This increases the conductivity between the silver oxide globules and results in lower picture resolution as well as sensitivity which falls below that required for commercial use. The conductivity between globules, for example, prevents the tube from distinguishing between close images of the optical pattern.

It is thus an object of this invention to provide a novel method for processing a photocathode of a pickup tube.

It is another object of the invention to provide a pickup tube of improved sensitivity.

It is another object of the inventionto provide a novel method for processing television pickup tubes which have lost their sensitivity due to extended use.

The invention is that of improving the photosensitivity of a television pickup tube, as well as, to provide a photocathode of greater sensitivity and picture resolution. It has been found that the loss of sensitivity or the low sensitivity of a pickup tube can be due to the deposition of cesium metal to form conductive paths between the photosensitive globules of the mosaic formed on a mica insulating sheet. In accordance with the invention, the cesium deposits on the insulating support sheet are removed by a process of positive ion bombardment coupled with a baking of the tube. A glow discharge is established within the tube envelope by means of external high frequency fields. The positive ion bombardment of the target sputters away particles of cesium from between the mosaic globules. This process is followed by the baking of the tube at around 200 C. to prevent the removed cesium metal from diffusing back upon the mosaic.

Fig. l discloses in section an iconoscope camera pickup tube in accordance with my invention.

Fig. 2 is a sectional view of the tube of Fig. 1 along line 2-2 of Fig. 1.

Fig. 1 discloses an iconoscope-type television pickup tube having an evacuated tubular envelope 10 with a neck portion 12 mounted at an angle to the axis of the tubular envelope portion. Mounted within the envelope neck portion 12 is an electron gun 14 for providing a beam of electrons which are accelerated and directed toward a target electrode 16. The target 16 is mounted transversely to the axis of the tubular neck portion 12, and substantially perpendicularly to the axis of the tubular envelope.

The electron gun is of a conventional type and consists of a cathode electrode formed of a short tubular element closed at the end closest to target 16 by an end wall which, in turn, is coated in a well-known manner with a mixture of barium and strontium oxides to provide a source of electron emission. Enclosing and supporting the tubular cathode 18 is a coaxial tubular control grid electrode 20 closed by an apertured wall portion 21 at its end closest to target 16, and as shown in Fig. l. Spaced along the axis of the tubular envelope portion 12 from control grid electrode 21 is a first tubular accelerating electrode 22 and a second and third accelerating electrodes 24 and 26 respectively. The first accelerating electrode 22 is closed at its end facing the target electrode 16 by a metal wall portion or disc 28 having a centrally-disposed aperture therethrough. Accelerating electrodes 24 and 26 are in the form of shallow thimbles or recessed discs or plates, each having a centrally-disposed aperture coaxial with the apertures in the electrode wall portions 28 and 21 respectively.

A conductive film 30 formed of any appropriate material, such as metal or a suspension of carbon particles in a binder, is coated on the inner surface of the envelope neck portion 12 from a point surrounding the end of the gun structure 14- into the tubular envelope portion to a point adjacent the target electrode 16 as is shown in Fig. l. he coating 30 is connected conductively to accelerating electrodes 26 and 2-2.

The several electrodes of the electron gun 1d are connected respectively, to several points of a voltage divider 32 to establish operating potentials on the electrodes. The potential values are indicated in Fig. l are not limiting, but are indicative of those which have been used successfully in a tube of the type described.

Electron emission from the coated surface of cathode 18 is formed into an electron beam, as is well-known, and is accelerated and directed through the several apertures of the gun electrodes. The beam passes along the axis of the tubular envelope portion 12 and is focused by gun 14 to a small spot on the target electrode 16. The electron beam is scanned over the surface of target 16 by magnetic fields in any conventional manner, as for example, by fields established between two pairs of coils mounted about the tube neck 12 in a yoke structure 34. The coils of each pair of scanning coils are connected in series to sources of sawtooth currents to provide line and frame scanning, respectively of the electron beam over :the surface of target 16.

As shown in Fig. 1 and in greater detail in Fig. 2, the target electrode consists of essentially a supporting plate 36 formed of some insulating material such as mica. .Riveted to the support plate 36 is a target assembly including an insulating sheet 38. On the surface of the insulating sheet 38 supporting a conductive film or coating 40 adjacent to the support plate 36. Film 40 may be of any appropriate material such as metal or a colloidal suspension of carbon particles in a binder.

On the opposite face of the target sheet 38, there is formed a photosensitive mosaic 42 which consists essentially of small islands or insulated particles of photosensitized silver oxide.

The operation of the tube shown in Fig. 1 is briefiy that in which the electron beam of gun 14 will strike the target 16 with an energy of substantially 1000 volts. This initiates from the target surface a secondary electron emission which drives the target surface under the beam positively to a potential of about three volts higher than the potential of the collector electrode 30. At this potential the secondary electrons leaving the target surface will pass partially to the collector electrode 30 and will partially be re-distributed to other portions of the target previously driven positively by the beam. The re-distribution electrons tend to drive these other portions of the target down to a potential of substantially 1 /2 volts minus with respect to collector potential 30.

An optical image is focused upon the mosaic surface 42 of the target electrode 16 through an optical glass window 11 of envelope by any appropriate lens system indicated at 47. The sensitized mosaic 42 will emit electrons photoelectrically from each portion of the target surface and in proportion to the amount of light striking that part. in this manner, there is established on target 16 a distribution of charges or a charge pattern corresponding to the optical image or light distribution focused upon the sensitized target surface.

The photoelectrons emitted from the mosaic 32 tend to raise the potential of the illuminated portions of the target surface 42 to a point somewhere between a minus 1 /2 volts and a plus 3 volts with respect to the potential of collector 30. As the electron beam is again scanned over the target surface, each portion of the target area struck by the electron beam is instantaneously brought to the 3 volts positive with respect to collector potential and from a potential which varies according to the amount of light falling upon that target portion. The instantaneous driving of each target portion positively by the beam, produces a current pulse in the signal plate 40, which is capacitively coupled to the mosaic surface of the target. The pulse is proportional to the amount that each area of the target is raised by the electron beam. The beam then in scanning the charge pattern on the target surface sets up a succession of current pulses which in turn provide corresponding voltage changes in the circuit 46 of the back plate 40, which is coupled to the control grid 48 of the amplifier tube circuit. In this manner, there is produced an output signal of the tube.

In tubes of the type described, the photocathode mosaic '42 can be formed by dusting on the target sheet 38 a coating of silver oxide. The coated sheet 38 is fired in a furnace to a temperature between 850 C. and 900 C. to reduce the silver oxide to silver or more likely to a silver-silver oxide eutectic. Although silver metal melts at 960 C., the eutectic formed at the firing temperature 'of around 900 C. tends to coalesce or form into small globules which become insulated from each other by the surface of sheet 38. The conductive film or coating 40 is next applied to the opposite side of target sheet.

Sheet 38 is riveted to the support plate 36, which in silver, and silver oxide is formed on the globules.

turn is mounted within envelope 10 by fastening support plate 36 to glass studs 44 sealed into the envelope wall. Target 16 is mounted perpendicular to the axis of the tubular envelope 10 and transversely to the path of the electron beam along the axis of envelope neck portion 12. A lead 46 is fixed to the conductive film 40 and brought out through the envelope wall, as schematically shown in Fig. 1.

Fixed to the tube envelope 10 is a closed tubulation 50 within which is frictionally fixed an evaporator support assembly 52 upon which are mounted a plurality of metal capsules 54 having a mixture of cesium chromate and silicon. When heated, the cesium chromate is reduced to cesium metal.

Also previously mounted Within the tube envelope are a pair of silver evaporators consisting of filament wires 56 mounted between a pair of conductive leads 58 as shown in Figure 2, for example. Fixed to each filament wire is a globule of pure silver metal. The silver evaporator filaments 56 are mounted one on each side of the target and close to the side walls of the tube envelope 10, so as to be out of the way of the optical picture focused onto the target 16 of the tube. Furthermore, the silver evaporators include a strip of mica 62 for shielding the envelope wall or window 11 from silver metal to be evaporated onto the target electrode.

The process of forming the photosensitive cathode 42 is that in which the tube 10, with the various tube parts described above mounted within the tube, is sealed to an exhaust system through a tubulation 64. The tube is slowly evacuated and then placed in an oven 'lfiCl baked for approximately 30 minutes at 450 C. The tube then processed in a conventional manner which includes heating the various metallic gun electrodes and other structures within the tube by a high frequency coil placed externally against the envelope wall and adjacent to the metal structure respectively. The tube is on exhaust during this baking and degassing operation so as to pump out of the tube envelope all occluded gases driven off of the tube structure by the heating and degassing steps. The tube after this is cooled to room temperature and oxygen is introduced through the tubulation 64 until the pressure within the tube envelope has risen to .020 mm. of mercury.

The silver globules in the mosaic are next oxidized by establishing a glow discharge within the tube envelope. This may be done as fully described in the U. S. Patent 2,020,305 to Essig. Normally, the oxidation discharge is provided by using a radio frequency wand in which the wand is an ungrounded electrode connected to one end of the high voltage coil of an R. F. transformer. Placing the wand adjacent the tube envelope sets up a glow discharge within the tube which can be applied adjacent to the target electrode 16 so that, in the presence of the oxygen within the tube 10, the silver metal globules 42 become oxidized. The glow discharge is continued until the surface of the target changes to a uniform grayblue color. The oxygen is pumped out of the tube and when the tube is evacuated, the cesium capsules 54 in the tubulation 50 are flashed by placing a R. F. coil coaxially to each capsule, whereby R. F. currents induced in the capsule vaporizes the reduced cesium metal in the capsule. The cesium vapor ruptures the capsule and escapes into the closed tubulation 50. Using a hand torch or burner, the cesium metal deposited on the walls of tubulation 50 is vaporized again and driven into the tube envelope 10.

Two Aquadag or graphite patches (not shown) painted on the internal wall of tube envelope 10, exhibit gettering action with respect to cesium vapor. A change from black to a reddish orange color in a portion of the patches is used to indicate a suflicient amount of cesium vapor within the tube.

The tube 10 is then baked for approximately 10 minutes at 200 C. until a photosensitive complex of cesium, The

tubulation 50 is then sealed off and removed from the tube 10. Also the exhaust tubulation 64 is tipped off by sealing the tube and disconnecting it from the exhaust system.

The tube is placed in a test set and an optical image or picture focused upon the target 16. Furthermore, all of the gun and target electrodes are connected into appropriate circuits and the tube is readied for operation in a normal manner. However, the cesium metal vapor within the tube has also deposited upon the target 16 and there probably will be little or no video output signal produced, since the silver oxide globules 42 are connected conductively one to another by the cesium metal. The tube 10 is taken out of the test set and baked for approximately 10 minutes at 180 C. to increase the resolution of the target surface 42.

The resolution of the photosensitive target is determined by the ability of the tube 10 to resolve details of the optical picture projected onto the target. The degree to which the globules of the mosaic 42 are insulated from each other enable each minute globule to hold its charge independently and to produce its independent video pulse. Conductive paths between globules prevent resolution of the details of the picture projected upon the connected globules. Baking of the tube causes the cesium from the target to evaporate and lessens the conductivity of the cesium paths between the silver oxide globules 42 and thus enables the mosaic to resolve picture details to a better degree. However, at the same time there is loss of sensitivity since the sensitizing cesium metal on the globules also is evaporated. The baking and testing is continued, however, until sufficient cesium metal is evaporated fro-m between the globules to give a sensitivity of 400 to 500 lines. This means that the resolution of the tube is that in which two solid vertical, black lines on the mosaic each 0.02 cm. wide and separated by a distance of 0.02 cm. can be reproduced and seen usually on a picture tube receiving the signal from tube 10. It is desirable that the resolution of the tube be greater than this, but further baking of the tube will cut down the sensitivity of the target too greatly. However, resolution of the target can be increased further without losing sensitivity, by evaporating silver onto the mosaic target surface. This isdone by passing a heating current through the silver evaporator filaments 56. The vaporized silver from globules 60 pass over to the target surface at relatively high velocity in the vacuum. The evaporated silver metal striking the target mosaic helps to break down the cesium metal paths between the globules 42. It is believed that the silver metal upon striking the mica surface tends to ball-up and drag the cesium film into the ball. In this manner or some other manner, however, the cesium film is broken up and the resolution is further increased without loss of sensitivity.

It has been described above, however, that the baking of the tube lowers the sensitivity of the target surface because of the vaporization of the cesium from the silver oxide globules. It is desirable to retain maximum sensitivity, however, with an increase in resolution. To do this it would be necessary to reduce the amount of baking of the tube. The evaporation of the silver provides a partial solution of the problem, but it has been found, in accordance with the invention, that the baking of the tube can be minimized and the cesium metal removed from between the silver oxide globules by a different procedure. In accordance, with the invention the tube after sealing 01f, as described above, is baked at around 180 to 200 C. for a period of 10 or more minutes to increase the resolution of the tube to an optimum value without materially reducing the sensitivity of the tube. The silver metal is evaporated onto the target from the filaments 56, as described above. Then, in accordance with the invention, a high frequency electrical field produced by a R. F. wand or electrode is established within the tube so that ionization of the residual gas particles within the tube causes a discharge to'take place adjacent the target electrode. The positive gas ions formed in the discharge will bombard the target electrode and physically dislodge or sputter away particles of cesium metal which lie between the photosensitive globules. This cesium sputters off and is deposited on adjacent surfaces such as the bulb walls, for example. This positive ion bombardment of the target does not dislodge the cesium from the silver oxide globules because the cesium is partially held captive by the globules. The positive ion bombardment of the target is continued until the desired sensitivity of the tube has increased to around 500 lines resolution. This can be determined by placing the tube into the test set and operating the tube with a signal pattern projected onto the photocathode. If the sensitivity is insufiicient, the tube can be removed and again subjected to the high frequency field to further increase the resolution of the tube. In order to prevent the cesium which has been removed by the positive ion bombardment from recondensing onto the target surface, the tube must be baked at around 200 C. The baking heats the glass walls to the baking temperature at which point they are more absorptive of the cesium deposited thereon and will tend to hold the cesium captive and prevent its redepositing upon the target during tube operation.

The above described process of increasing tube resolution and target sensitivity may also be used to salvage pickup tubes of the type described which have lost both their signal strength and resolution because excess ccsium within the tube bulb has migrated back onto the mosaic target during tube operation. This redeposited cesium also tends to form conductive paths between the globules of the mosaic and shorts-out the target surface, as described above. The tube to be salvaged is subjected to the high frequency electrical field, as described above, for providing a positive ion bombardment of the target surface. This process breaks down the cesium film between the mosaic particles and thus increases the resolution and restores the sensitivity of the target. This is also followed by a baking step to fix the cesium on the tube walls.

What is claimed is:

1. The method of processing a photosensitive mosaic electrode formed of a coating of silver oxide globules coated with a deposit of cesium, said method comprising the step of, exposing the silver oxide coating to a positive gaseous ion bombardment to decrease the electrical conductivity between the globules of the mosaic.

2. The method of processing a photosensitive mosaic electrode formed of-a coating of silver oxide globules coated with a deposit of cesium, said method comprising the steps of, evaporating silver metal in vacuum onto the mosaic coating, and exposing the silver oxide coating to a positive gaseous ion bombardment to decrease the electrical conductivity between the globules of the mosaic.

3. The method of processing a photosensitive mosaic electrode mounted within an evacuated glass tube envelope, said electrode including a coating of silver oxide globules on one surface of an insulating support and coated with a deposit of cesium metal, said method cornprising the steps of, exposing the silver oxide coating to a positive gaseous ion bombardment to decrease the electrical conductivity between the globules of the mosaic, and baking said tube to a temperature at which the glass walls of said tube absorb cesium metal driven onto the glass walls by the positive ion bombardment.

4.The method of processing a photosensitive mosaic electrode mounted Within an evacuated glass tube envelope, said electrode including a coating of silver oxide globules On one surface of an insulating support and coated with a deposit of cesium metal, said method comprising the steps of, exposing the silver oxide coating to a positive ion bombardment to decrease the electrical conductivity between the globules of the mosaic, and baking said tube at around 200 C. to cause the glass tube positive gaseous ion bombardment to decrease the elec- 10 2,463,180

-trical conductivity between the globules of the mosaic,

and baking said tube at around 200 C. to cause the glass tube walls to absorb cesium metal driven onto the walls by the positive ion bombardment.

References Cited in the file of this patent UNITED STATES PATENTS 2,178,233 Klatzow Oct. 31, 1939 2,189,986 Hickok Feb. 13, 1940 Johnson Mar. 1, 1949 

1. THE METHOD OF PROCESSING A PHOTOSENSITIVE MOSAIC ELECTRODE FORMED OF A COATING OF SILVER OXIDE GLOBULES COATED WITH A DEPOSIT OF CESIUM, SAID METHOD COMPRISING THE STEP OF, EXPOSING THE SILVER OXIDE COATING TO A POSITIVE GASEOUS ION BOMBARDMENT TO DECREASE THE ELECTRICAL CONDUCTIVITY BETWEEN THE GLOBULES OF THE MOSAIC. 